Since you're throwing the party, you get to make the rules. You get to decide if you want the guests to be dressed up or not. An afternoon tea generally means you want your guests to be comfortable but a little dressy. Nice pants and a blazer, a skirt and sweater. Most people get that, but let me know what you want them to wear and I'll say it for you.
NOW! You can throw any kind of shower that you want as well. Most showers are "general" ones, but "themed" ones are becoming more and more popular. So what you suggest in your question is the EXACT right thing. Instead of a "Kitchen Shower" or "Landscaping Shower" where the focus is on one area of the future bride's home, you can do a "Lingerie Shower". It's no more complicated that saying on the invitation "You're invited to a lingerie party for Lisa". That lets the guests know what they are expected to bring. If you want to be clever of funny about it I would find a "conversion" chart.
You know how an American size 9 shoe is called a 6.5 in the UK, a 40.5 in "EU" terms and a 26 in CM, whatever that means? I am sure there are charts on the internet mapping this out. Dot it with tons of
question marks and word the party invitation as if YOU ALL do not get her nightgowns, the poor dear will have none, because who can figure these European sizes OUT? Just a suggestion.
Another thing you can do, see how you feel about it, is have everyone offer to send her a care package. Each guests takes a different month in the future, writes out a card letting her know who to expect to hear from in March of 2005, for instance. If I were moving to Europe, I would probably like to get stuff from "home". Maybe it will just be pictures from someone's Christmas or vacation, or maybe a box with Kraft Macaroni and Cheese or Brownie mix. I have a friend living in Tokyo and you would be surprised at the odd things he misses and desires sent.
This alternative is risky, everyone is promising to do something in the future, and hopefully they're following through. And postage can be expensive to Europe, even if the items you're sending are not.
The theme bridal showers are common practice and your idea sounds quite thoughtful and romantic, while at the same time being practical. Simply state on the invitation what the theme of the shower is. When people call to accept, you can give them some suggestions of what to give as a gift.
Since the shower is also a tea party, the women should be dressed up a bit. You can also state the claimed attire on the invitation. It could read something like, "Please wear your Sunday best for the Tea Party Shower.
Having family and friends there for you and going through everything with them makes you thank god that you had family and friends that love you and just want what is best for you and the soon to be little one. Having people that went despite what you are going though at your baby shower and talking to them about your plans for your little one and talking about the names that you and your husband are thinking about is something that you will always remember.